Add Bazel support

[Vertexwahn]

This PR helps users of the Bazel build system to integrate Embree in their builds.

Supporting two build systems (CMake and Bazel) is more effort to maintain but helps spread Embree.
In the worst case the Bazel build does simply not work - in the best case, someone who uses Bazel can benefit from it.

Also, other projects support CMake and Bazel side by side (e.g. Catch2, GoogleTest, oneTBB, https://github.com/AcademySoftwareFoundation/openexr, etc.).

I am willing to support the Bazel build support of Embree at least for a half year - after this period I can reconsider doing it for another half year

[Vertexwahn]

@svenwoop Can you have a look?

[svenwoop]

We discussed this internally and we do not want to add other build systems other than cmake to Embree.

[Vertexwahn]

Finally, I came up now with rules_embree.